Transducer system



` pms, 1946. A HJ. wou.A 2,412,023

TRANSDUCER SYSTEM Filed Dec. 1, 1944 INVENTOR ATTORNEY Patented Dec. 3, 1946 TRANSDUCER SYSTEM Harry J. Woll, Indianapolis, Ind., assigner to Radio Corporation of America, a corporation of Delaware Application December 1, 1944, Serial No. 566,143

My present invention relates generally to an improved balanced transducer system, and more particularly to a novel system for deriving from small variations of electrical capacity into correspondingly relatively large variations of direct current or potential.

In his application Serial No. 490,614, filed June l2, 1943, U. S. Patent No. 2,371,373, issued March 13, 1945, A. Badmaieff has disclosed and claimed a method of, and means for, using a balanced transducer for concurrently varying the frequency of a source of oscillations and a discriminator reso-nant frequency in opposite senses thereby to provide cancellation of even harmonics, greatly to reduce distortion and to double the discriminator output voltage. More specifically, Badmaieff disclosed an embodiment wherein a push-pull form oi condenser pickup concurrently, but in opposite senses, varied the frequency of the oscillator tank circuit of a multigrid tube and the frequency of a discriminator circuit located in the plate circuit of the multigrid tube. He utilized a rectifier to derive from the plate circuit a voltage representative of the pick-up capacity variations.

t is one of the important objects of my present invention to provide a more compact system having operational advantages similar to those of the Badmaieff system, but eliminating the need for a special rectifier.

Another important object of my invention is to assign a double function to the oscillator grid circuit of the aforesaid multi-grid tube; the oscillator grid circuit in my invention acting to provide grid-leak detection whereby the plate circuit has current flowing therethrough containing an audio frequency component.

Another, and more specific, object of this present invention is to provide a multi-grid tube having a cathode and two cold electrodes adaptedA to function as a generator of high frequency oscillations, whereas a cold output electrode is provided With a resonant circuit mistuned from the oscillator circuit by a predetermined amount, and a balanced reactive transducer being employed to vary the frequencies of the oscillator circuit and resonant output circuit whereby the latter develops a voltage whose magnitude and phase are dependent upon the oscillator frequency and the resonant output circuit frequency.

Still other objects of my invention are to improve the simplicity, eiiiciency and reliability of record reproduction systems of the frequency modulation (FM) type, and more especially to and 'provide a nigh fidelity,

tion record reproducer system.

- Still other features ef my invention will best be understood by reference to the-following description, taken in connection with the drawing, in which I have indicated diagrammatically a circuit organization whereby my invention may be carried into effect. l

In the drawing:

Fig. 1 shows an embodiment of my invention;

Fig. 2 shows an idealresonance curve of the resonant plate circuit. v

Reference is made to the aforesaid Badmaieff patent for a detailed explanation of the advari- Y connection with record reproduction, it is to beA clearly understood that it may be used for record cutting, measurement devices, Where linearity and Ismall pickup area are desired, such asfor pressure measurement, liquid height measurement,` elongation measurement, vibration measurement. Where the pickup device is a microphone, the system could be used for hearing-aid apparatus, public address systems, binaural systems and the like. v

The pickup device may be constructed in the manner shown by Badmaieif in his aforesaid patent, or it may be constructed in any other suit- 'able manner to provide push-pull reactance changes. The pickup is, therefore, schematically represented as comprising a pair of spaced fixedv electrodes and 2. Midway between the stators I and 2 is located a mobile electrode 3 provided with a record-scanning stylus. The circuit operatively associated with the pickup device willY now be described.

The tube 4 in Fig. l is shown as of the pentode type, although any other type tube may be used. Itsrcathode 5 is connected to a low alternating potential tap 5 on the oscillator coil Ll. The coil Ll is preferably. of the Variable inductance type, and has an adjustable core of comminuted iron. The core is schematically shown. The control grid l, which functions asthe oscillator section grid, is connected `through the direct `current blockingcondenser 8 to the high alternating potential side of coil L! and to xed plate I of the balanced capacity pickup device. The mobile element 3 of the pickup device is Aconnected by lead 9 to the negative terminal B, which may be ground,V of the direct current energizing source,

low cost and low distor The positive terminal +B of the source may be at +300 volts. The positive terminal +B is connected through resistor S to the cold electrode I of multi-grid tube 4.

Electrode I0 functions as the oscillator anode of the tube. The output electrode or plate Il has one connection through the directcurrent blocking condenser I2 to the fixed plate or stator 2 of the balanced capacity pickup device. A second conductor connects plate II to the high alternating potential end o! the discriminator coil L2 in accordance with my invention. The coil L2 is, also, a variable inductance having a core of comminuted iron, and the low potential end of coil L2 is connected to the +B terminal through output load resistor I 3. The third or suppressor grid I4 of tube 4 may be connected to grounded lead 9. A suitable bypass condenser 9" is connected to ground from the energizing lead feeding the anode I 0 and plate II. Conldenser I 0 connects the junction of coil L2 and resistor I3 to ground. Condenser I0 establishes the lower end of coil L2 at ground potential for high frequency currents, and has a relatively high impedance to audio frequency currents.

The cathode 5, grid 1 and electrode I IJ of tube 4 provide the oscillator section of the system. The positive cold electrode I0 is connected to the lower end of coil LI through direct current blocking condenser I5. Electrode I0 is, therefore, the oscillator anode. Coil LI is resonated to a mean, or center, frequency Fc. This mean frequency is to be deviated in accordance with the amplitude of the modulation frequencies. The mean frequency of the oscillator may be chosen from a range of 20 to 60 megacycles (mc.) The deviation range of the mean frequency is dependent on the capacity change and the mean frequency employed. A deviation up to about 1% of the mean frequency could be employed, but it should be noted that this total deviation is the sum of the oscillator deviation and the discriminator resonance peak deviation. The choice of the mean frequency of the oscillator is dependent on the sensitivity that is required from the pickup device. Since the capacity changeQdue to vibration, is of constant range as compared to the` 4 quencies disclosed herein.

' It will be understood that in accordance with my present method the condenser I, 3 is connected across the oscillator coil LI, while the.

condenser 3. 2 is shunted across the discriminator coil L2. The discriminator coil should be slightly off tune from the oscillator mean frequency Fc. either above or below. For example, the oscillator mean frequency, as shown in Fig. 2, may be below the discriminator coil peak resonance frequency. The amount of off tune depends on the "Q of the discriminator resonance, which is the slope of the two sides of the peak. The oscillator mean frequency Fc should be located at a fairl"7 linear part of the discriminator curve side. The maximum frequency deviation could be as high as 120 kilocycles (kc.) on either side of Fc. The generic expression "angle modulated is used herein to designate that the modulation of the high frequency oscillations may either be in frequency or phase. Preferably. it is desirable to adjust the iron core of coil L2 so that the peak frequency of the discriminator circuit is of a value such that the mean oscillator frequency Fc falls at approximately '10% of the response on one side of the resonance curve characteristic of the discriminator circuit. Specifically, and merely by way of illustration, Fc could be 26 mc., while the plate coil L2 could be normally tuned to 26.1 mc.

The oscillator grid condenser 8 is shunted by grid leak resistor I6 thereby providing the means for grid-leak detection at the oscillator grid. The numeral I1 indicates a coupling path from the plate circuit of tube 4 to the oscillator grid circuit. The coupling means may be an external reactance, or it may be provided by the distributed capacity between the pickup plates I and 2 as well as the inter-electrode capacity within the tube.

In general, the operation of the system is such as to develop across load resistor I3 a signal voltage representative of the displacement of electrode 3. Where the latter is actuated by a stylus scanning the grooves of a sound record, there will be developed across resistor I3 amplied audio frequency voltage corresponding to the recorded sound. Variation of capacities of condensers I, 3 and 3, 2 changes the tuning of the tuned circuitSLI-L 3 and L2-2, 3 with respect to each other. If the plate circuit L2-2, 3 and oscillator grid circuit LI-I, 3 were tuned to a common frequency, plate voltage would be fed back to `grid 1 in phase quadrature with the oscillator grid voltage. The feedback occurs, of course, through path I1. If, however, the two resonant circuits are not tuned to a common frequency, the plate voltage will be fed back to oscillator grid 1 at a phase angle greater or less than degrees depending on the sense of detuning. AS a result of this feedback voltage the oscillator voltage will be dependent upon the relative tunings of the two resonant circuits. Since the oscillator grid bias depends upon the amplitude of oscillations, the plate current of tube 4 is also dependent on the relative detuning of the two tuned circuits. Hence, the variable plate current develops a variable voltage across resistor I3 which is a measure of the motion of mobile electrode 3.

While the invention is in no way limited or restricted to actual constants, the following are supplied by way of illustration:

Resistance of resistor I6=18.000 ohms Resistance of resistor 9=41,000 ohms Resistance of resistor I 3=100,000 ohms Capacity of condenser 8:27 micro microfarads (mmf.)

Capacity of condenser I5=120 mmf.

Capacity of condenser I0=33 mmf.

Inductance of coil LI=20 turns at 30 mc.

Inductance of coil L2=20 turns at 30 mc.

Considering more specifically the electrical relations existing in the present system, it is pointed out that the locally-produced oscillations of 26 mc. are both frequency modulated and amplitude modulated. In my present system I utilize the fact that due to the non-linear action of the oscillator, the plate current 0f tube 4 Will contain an audio frequency component. It is the voltage fed back through coupling path I1 which amplitude modulates the high frequency oscillations. However, it is the frequency modulating of the oscillations which causes the plate circuit L2-2,

anaozs 3 to develop a voltage whose magnitude and phase are variable and depend on the oscillator frequency. The oscillator section excites the plate resonant circuit by virtue of electron coupling within the tube. When the high frequency plate current flows through its resonant circuit, the voltage developed thereacross depends upon what the oscillator frequency is at that moment with respect to the frequency of resonance of the resonant plate circuit. It will be obvious, therefore, that wide changes in phase and magnitude of the high frequency plate voltage will be developed because of the push-pull detuning of the oscillator tank circuit and the plate resonant circuit. As the mobile electrode 3 moves in one direction the frequency of oscillation is varied. Concurrently the frequency of the plate resonant circuit is changed in an opposite sense thereby causing the high frequency voltage across L2 to change to a degree determined by both of the said changes.

Since there exists coupling between the two resonant circuits (and this coupling may be capacitative, inductive, resistive or space charge) voltage is fed back from coil L2 to the oscillator grid v`I. There is, therefore, produced a secondary change in oscillator frequency and amplitude due to this feedback voltage. This secondary change in amplitude of oscillation affects the plate current flow in the tube, and produces an audio frequency voltage across the load resistor I3. It

may, also, be said that the leaky grid condenser,

At the same time the quency oscillation changes at an audio rate, the

grid current, and hence the grid voltage, will have an audio component. The grid, therefore, has a radio frequency voltage, an audio voltage, and a direct current voltage applied to it. The direct current voltage serves as a bias for the tube. The radio frequency voltage and audio voltage are amplified in the tube, and appear across the im pedances in the plate and screen circuits. The plate tank consisting of L2 and stray capacities has appreciable impedance only at the radio frequency, so only the radio frequency voltage appears across it. The resistor I3 is bypassed for radio frequency so only the audio voltage appears across it. When an amplitude modulated radio frequency voltage is applied to a grid-leak detector, the grid circuit acts exactly as a diode detector thereby resulting in the detected audio voltage being present between grid and cathode of the tube. This voltage is amplified by the tube, and appears in the plate circuit. The entire processI is called grid-leak detection.

In practice, the major portion of the tuning capacity for coil LI is contributed by stray capacitance's, and especially that of the leads .from the oscillator section to the pickup path, Merely by way of illustration, it is pointed out that in an average pickup the capacity between pickup electrodes is about 0.25 mmf., while the capacity between wires is of the order of 10 mmf.

At coil L2 the stray capacity of the wiringl must be considered part of the tuning capacity.

It is4 pointed out, furthermore, thatr either of the fixed electrodes I or- 2 may be removed thereby to provide single-sided operation. If the associated wire is removed, a xed capacitor should replace the stray capacities which would be absent. In the case of single-sidedy operation the. general operation will be the same, except that the variations in phase and magnitude of the high frequency voltage across coil L2 will not be as great as in the case of push-pull operation.

While I have indicated and described a system for carrying my invention into effect, it will be apparentto one skilled in the art that my invention is by no means limited .to the particular organization shown and described, but that many modifications may be made without departing from the scope of my invention.

What I claim is:

l. In combination, a tube provided with an oscillator section and an output electrode, means connected -to said oscillator section to provide angle modulated oscillations which have a predetermined mean frequency, a'resonant circuit coupled to said output electrode, electron coupling between said oscillator section and said output electrode developing said angle modulated oscillations in the circuit connected to said output electrode, said resonant circuit being tuned to a frequency different from said mean frequency by a predetermined value, means responsive to operation of saidrfirst-named means for providing frequency changes at the output electrode resonant circuit, a feedback coupling from the resonant output circuit to said oscillator section, and resistive means in circuit with the output electrode for developing a Voltage representative of theV said angle modulation.

2. In combination, a tube provided with an oscillator section'and a positive output electrode, transducer means connected to said oscillator section to provide frequency modulated oscillations which have a predetermined mean frequency, a resonant circuit coupled toxsaid output electrode, electron coupling between said oscillator section and said output electrode developing oscillations in the circuit connected to said output electrode, said resonant circuit being tuned to a frequency different from said mean frequency, a second transducer means responsive to operation of said first-named transducer means for providing changes in tuning of said resonant output circuit, a capacity feedback path coupling the output electrode to said oscillator section, and an audio frequency voltage output resistor in circuit` with the output electrode.

3. In combination, a tube provided with an oscillator section and an output electrode, means connected to said oscillator section to provide modulated oscillations which have a predetermined mean frequency, a resonant circuit coupled to said output electrode, coupling between said oscillator section and said output electrode developing said modulated oscillations in the circuit connected to said output electrode, means responsive to operation of said first-named means for providing frequency changes at the output electrode resonant circuit, a feedback path from the resonant output circuit to said oscillator section, and means in circuit with the output electrode for developing a voltage representative of the said modulation.

4. In combination, a tube provided with an oscillator section and a positive output electrode,

transducer means connected to said oscillator section to provide frequency modulated oscillationsv which have a predetermined mean frequency, a resonant circuit coupled to said output electrode, coupling between said oscillator section and said output electrode for developing oscillations in the circuit connected to said output electrode, a feedback path coupling the output electrode to said oscillator section, and an audio frequency output resistor in circuit with the output electrode.

5. In a sound record reproducer system, a tube provided with an oscillator section and an outputele'ctrode, means'connectedvto said oscillator' section `to provide angle modulated oscillations which have a predetermined mean frequency, a resonant circuit coupled to said output electrode, electron coupling between said oscillator section and said output electrode for developing said angle modulated oscillations inthe circuit'connecte'd to said output electrode, said resonant circuit being tuned to a frequency different from said mean frequency by a predetermined value, a feedback coupling from the resonant output circuit to said oscillator section, grid-leak detection means in circuit with the oscillator section grid, and resistive means in. circuit with the output electrode for developing a voltage representative of the said modulation.

6. In combination in a reproducer of sound records, a tube provided with an oscillator section and a positive output electrode, capacity pickup means connected to said oscillator section to provide frequency modulated oscillations which have. a predetermined mean frequency, a resonant circuit coupled to said output electrode, electron coupling between said oscillator section andY said output electrode for developing oscillations in the circuit connected to said output electrode, said resonant circuit being tuned toa frequency different from said mean frequency, a second capacity pickup means responsive to operation of u'said rstenamed pickup means for providing changes in tuning of said resonant output circuit, a capacity feedback path coupling the output electrode to said oscillator section, and an audio 4frequencyV output resistor in circuit with'the out` putelectrode. f v

7. In combination,l in a system for reproducing records,'a tube provided with an oscillator section and an output electrode, said oscillator section predetermined meanfrequency, a resonant cir cuit coupled to said -output electrode, electron coupling between said oscillator section 'and said output electrode for developing saidfrequency modulated oscillationsin the circuit connected to said output electrode, said resonant circuitbeing tuned to a frequency different from said mean frequency-by a predetermined value, means responsive to operation of said first-named means for providing frequency changes at the output electrode resonant circuit, a feedback coupling from the resonant output circuit to said oscillator grid, grid leak detection means in circuit with said oscillator grid, and resistive means in circuit with the output electrode resonant circuit for developing an audio frequency voltage representative of the said modulation.

8. In combination, a tube provided with an oscillator section and a positive output electrode, a rst capacity pickup device connected to said oscillator section to provide frequency modulated I,oscillations which have a predetermined mean frequency of the order of 26 megacycles, a resonant circuit coupled to said output electrode, electron coupling between said oscillator section `and said output electrode developing oscillations in the circuit connected to said output electrode, said resonant circuit being tuned to a frequency of the order of 26.1 megacycles, a second capacity pickup device responsive to operation of said firstnamed pickup device for providing opposite changesl in tuning of resonant output circuit, a capacity feedback path coupling the output electrode to said oscillator section, and an audio frequency output resistor in circuit with the output electrode.

HARRY J. WOLL. 

